The present invention relates to a signal generating circuit and a related method, and more particularly to a modulator capable of calibrating a gain mismatch of a controllable oscillator in the modulator and a related method.
Conventionally, a two-point modulator (TPM) is a low-complexity implementation of a transmitter concept for transceivers in mobile radio systems. The two-point modulator generally comprises a phase-locked loop (PLL) circuit, a low-pass modulating path, and a high-pass modulating path, in which the input modulation data is fed to the phase-locked loop circuit via the low-pass modulating path and the high-pass modulating path. Generally, the low-pass modulating path is connected to the feedback divider of the phase-locked loop circuit, and the high-pass modulating path is connected to the voltage-controlled oscillator (VCO) of the phase-locked loop circuit. Ideally, the two-point modulator has an all-pass response from the input modulation data to the output signal of the voltage-controlled oscillator, so that the phase-locked loop circuit has a transmission response which is independent of frequency. However, as the gain of the voltage-controlled oscillator is a variable parameter with respect to the frequency, the voltage-controlled oscillator may induce a gain mismatch when the frequency band of the transmitted output signal is changed. In other words, the frequency response of the two-point modulator deteriorates when the two-point modulator is used to transmit a multi-band output signal. Therefore, providing an efficient way to detect and calibrate the gain mismatch problem that emerges in the two-point modulator is a significant concern in the mobile radio field.